I’d suggest you steer clear of SWC designs and use either RNFP or TC style pills.
Marlins have a reputation of having large groove diameters. It would be worth your while to slug the barrel to determine the diameter, then add 2 thou. There are a number of mould suppliers now producing 0.432 and 0.433 moulds. Most of the big names produce moulds that generally run 0.430.
It’s worth noting that pistols in 44 have tighter barrel specs and a faster twist. Check the SAAMI specs for 44mag rifle and pistol - you’d think they were totally different cartridges.
